Mac Mini M1 + Apple Cinema Display 30" - anyone got it working?

[zoltm]

I'd like to followup and say that the adaptor Ricardo.Dawson recommended worked great for me with my M1 Macbook Air and 30" cinema display at full resolution. Nothing else had worked so far. Thank you!

@HappyPig Just wanna confirm that you are having MBA -> JSAUX USB-C to Mini DP connector -> Apple Mini DP to Dual Link DVI -> ACD 30"

Thanks and cheers!

 

[pkamb]

ATP podcast this week discussed how the HDMI port on the new M1 Pro/Max MacBook Pros (and I assume the existing M1 Mac Mini) is internally using a Display Port > HDMI adapter.

Is there a way to adapt the 30" Cinema Display Dual Link DVI to the HDMI ports on the Mac Pros or Mini? Any drawbacks?

Maybe that would be the way to go, rather than using up a USB-C / Thunderbolt slot.

 

[zoltm]

ATP podcast this week discussed how the HDMI port on the new M1 Pro/Max MacBook Pros (and I assume the existing M1 Mac Mini) is internally using a Display Port > HDMI adapter.

Is there a way to adapt the 30" Cinema Display Dual Link DVI to the HDMI ports on the Mac Pros or Mini? Any drawbacks?

Maybe that would be the way to go, rather than using up a USB-C / Thunderbolt slot.

@pkamb
As far as I recall there is a cohort @winna here in this thread manage to run Dual ACD 30 with the M1 Mac Mini, 1 USB-C port + 1 HDMI port in the Mac Mini. The solution is kind of convoluted it requires to run HDMI with a powered HDMI -> Displayport convertor then run a Displayport to dual link DVI adaptor! But it does work.

HDMI:
https://www.amazon.com/dp/B07T16LLB8/ref=cm_sw_r_awdo_navT_a_J3GCR5J7JBFJPQF9W586

+

https://www.amazon.com/dp/B0746NCD1T/ref=cm_sw_r_awdo_navT_a_6HVHM5WSTF083NBJAF16

 

[gugy]

I've got my 30"/23" ACD to work on my M1 Max using the caldigit element hub connected to these adapters. They seem to be the most stable ones out of a few I bought at Amazon.

for the ACD 23 inch:

CableCreation USB C to Dual Link DVI Active Adapter, USB Type C to DVI-I Adapter 2560 x 1600 Resolution USB-C to DVI Signal Fomat Converter

The USB-C to Dual-Link DVI signal format converter seamlessly connect Type-C PC/Laptop or Mobile Phone to conventional DVI displays, such as CRT, LCD monitors and projectors, especially for 30-inch Cinema Display, you can enjoy the ultimate widescreen canvas with a resolution of 2560 x 1600 pixel...

www.amazon.com

For the 30 inch:

Club3D CAC-1510-A USB Type C to Dual Link DVI-D Cable Adapter Support 3840x2160@30hz, 2560x1600@60Hz HDCP Off for Apple Cinema Displays M/F

USB C to DVI-D Dualink adapter without HDCP Support for certain Apple Cinema monitors without HDCP The USB Type C to Dual Link DVI or Single Link DVI Adapter is the easiest solution for connecting your USB Type C source device featuring DisplayPort Alternate Mode to a DVI enabled Monitor or Proje...

www.amazon.com

I hope this helps someone. Cheers.

 

[Diff.eren.t]

I’ve been running an M1 mini with an ACD 30 for about six weeks now. I already had the Apple miniDP to dual link DVI adapter, and I got a USB-C to miniDP adapter from amazon and daisy chained them together.
Occasionally it won‘t wake from sleep and I need to unplug the USB-C adapter but apart from that it works fine.

Hello

MiniDP to dual link DVI adapter and USB-C to miniDP
One of them will be enough to get full ACD resolution ( 2560 x 1600 ) ?
If it is. Which model or brand will match precisely ?

Thanks a lot

 

[MiniMaxHouss]

Reporting back with my results.

First I'll note that I'm connecting to my M1 via a CalDigit dock that has DisplayPort and Thunderbolt inputs, so I basically did this along two tracks: DisplayPort and USB-C, all also including the old Apple DVI to Mini DisplayPort adapter, which I got off eBay for about $80 new in box.

Great news: I quickly found solutions that worked in both tracks!

Working products:

JSAUX Mini DisplayPort (F) to USB-C (M): https://www.amazon.com/dp/B0836HGBD7?psc=1&ref=ppx_yo2_dt_b_product_details

StarTech Mini DisplayPort (F) to DisplayPort (M): https://www.amazon.com/dp/B003N3DTKY?psc=1&ref=ppx_yo2_dt_b_product_details

Cable Matters Mini DisplayPort (F) to DisplayPort (M): https://www.amazon.com/dp/B014DV2RGG?psc=1&ref=ppx_yo2_dt_b_product_details

Non-working product:

Amiroko Mini DisplayPort (F) to USB-C (M): https://www.amazon.com/dp/B07DRQYWPD?psc=1&ref=ppx_yo2_dt_b_product_details

Last thoughts:

I have a Windows XP bootcamp install I use for old games on my very old MBP w/ Mini DisplayPort. I noticed a tiny bit of blue noise in dark areas under XP that I haven't yet detected in any other environment or on any other computer. That got me sweating again but I think it's confined to XP.

The scarcity of the Apple DVI-to-mDP adapter means that a perfect DVI-to-USB-C converter is still the ideal. For those who continue testing DVI-to-USB-C converters in particular: you must test by setting the desktop background to a black screen and looking for noise.

HI !

How can I know if my adaptor will work based on your experience ? I have this one : https://www.amazon.ca/dp/B072KRL6TH/ref=cm_sw_em_r_mt_dp_F5P0BYMPGEXG7ZS2FNT1?_encoding=UTF8&psc=1

it seems to have the same specs as the Amiroko that you mentioned, but then all seem to have the same specs…or I an missing something ?

 

[erikkfi]

HI !

How can I know if my adaptor will work based on your experience ? I have this one : https://www.amazon.ca/dp/B072KRL6TH/ref=cm_sw_em_r_mt_dp_F5P0BYMPGEXG7ZS2FNT1?_encoding=UTF8&psc=1

it seems to have the same specs as the Amiroko that you mentioned, but then all seem to have the same specs…or I an missing something ?

Hard to say without testing, I’m afraid. That Amiroko one simply didn’t work. Maybe it wasn’t the right spec or maybe it was just a defective single item.

If you can’t get one of the ones that’s been tested working, I’d say just make sure you have the right adapter chain for your setup and give one a go.

 

[MiniMaxHouss]

Thanks for your answer.

I ordered the JSAUX as many people say it works well, and I will try the MBP M1 Max -> JSAUX -> Apple Dual Link DVI to Mini Display port -> ACD 30". I really hope it works.

 

[Edge]

Just thought I'd add my experience here.

Not a Mac mini, but a MacBook Pro 16" M1 Pro connected to Apple Mini DisplayPort to Dual-DVI adapter via several options;

First I purchased a generic AUD$35 USB-C -> Mini DisplayPort Adapter (Comsol brand https://www.comsol.com.au/KCMDAD, but appears to be a rebadged product) and a generic USB-C -> USB-A adapter to power the Apple adapter. This worked, however the only downside was it would not wake from sleep without replugging the display cable.

I have just acquired an A-Logic Ultra USB-C Dock Plus v2 hub with a Mini DisplayPort, for AUD$159.
https://www.jbhifi.com.au/products/alogic-ultra-usb-c-dock-plus-v2
It works, and supports 100W USB-PD. With a USB-PD 3.0 compliant cable it charges the laptop nearly as fast as the MagSafe cable (both via the Apple 140W charger). It reports various VIA chipsets (and Realtek network chipset)

However, when I plug it in, the 30" Cinema Display flickers on and off every second or so, up to 10 times. It's quite disconcerting, but once it stops the monitor stays connected and the image is consistent. (This may be an issue with the 12 yr-old Cinema Display power brick.)

The A-Logic also seems to wake the Cinema Display from sleep, unlike the cheap USB-C -> mDP adapter. But I will need longer term data to confirm this.

But, the hub/dock gets super HOT. And stays HOT, irrespective of whether the laptop is charging, or even whether the hub/dock is plugged into the charger. It gets hot even when nothing is connected to it, just plugged into the Macbook. I may have to return it, as the temperatures seem abnormally high. There's a thread here discussing similar issues https://forums.macrumors.com/threads/usb-c-hub-gets-very-hot-on-macbook-pro-2020.2238109/

With its support for dual displays at up to 4K (60Hz) via its Mini DisplayPort and HDMI ports, the Alogic USB-C Ultra Dock PLUS Gen 2 Docking Station allows for easily creating a convenient multi-monitor workspace for your compatible device. Providing up to 100W of power delivery and featuring support for USB PD 3.0 standards, the Alogic Ultra Dock PLUS Gen 2 Docking Station can quickly and conveniently recharge various compatible devices. Built with a USB-C 3.2 and 2 USB-A 3.2 ports, this Ultra Dock PLUS Gen 2 Docking Station can easily accommodate multiple USB-enabled accessories. It also has SD and microSD card readers for handling memory cards and a 1Gbps Ethernet port for wired network connectivity. Compatible with various laptops, including Windows, Mac, and Chromebooks, this Alogic Docking Station is a practical option for various workstation setups. Its also Plug-and-Play compatible, which removes the hassle of manually installing drivers and docks

 

[EugW]

My 2007 30" Cinema Display supports HDCP. However, in Monterey on my 2014 Mac mini with mini-DP to DL-DVI adapter, Disney+ and Prime Video both stopped working in Safari, although Netflix still worked. I finally loaded up the Prime Video app and that didn't work either, telling me that my machine didn't support HDCP. (Curious that Netflix worked though. Maybe it was the content? I was watching a foreign cooking show.)

I tried rebooting the Mac mini, with no luck. I then tried turning off and on the monitor, again with no luck. Finally I unplugged the monitor from the Mac and plugged it back in. Success!

So, for some reason my setup lots its HDCP handshake, and the only way to get it back again was to physically unplug the monitor. Even a reboot of the Mac with the 30" ACD still plugged-in wasn't enough to renegotiate the HDCP handshake.

Anyone encounter this with their Apple Silicon Macs and the various adapters you're using?

P.S. Chrome continued to work with these streaming sites even when they weren't working in Safari, but I believe Chrome is always limited to 720p, whereas Safari supports up to 1080p with HDCP on this monitor.

 

[ACD30"]

You can’t use both USB-C ports. HDMI + USB-C

USB-C:
https://www.amazon.com/dp/B08B7FSVTL/ref=cm_sw_r_awdo_navT_a_MWAN5HXYNV2S9PA268Z9


HDMI:
https://www.amazon.com/dp/B07T16LLB8/ref=cm_sw_r_awdo_navT_a_J3GCR5J7JBFJPQF9W586

+

https://www.amazon.com/dp/B0746NCD1T/ref=cm_sw_r_awdo_navT_a_6HVHM5WSTF083NBJAF16

I notice that the latter two adapters you linked are each powered through USB-A. The Mac mini has enough ports to accommodate those adapters, but that leaves both USB-A ports occupied. My question is — do you have a USB-A hub attached to the Mac mini, for additional devices? Or, do you simply not require any other USB-A devices to begin with?

 

[EugW]

I notice that the latter two adapters you linked are each powered through USB-A. The Mac mini has enough ports to accommodate those adapters, but that leaves both USB-A ports occupied. My question is — do you have a USB-A hub attached to the Mac mini, for additional devices? Or, do you simply not require any other USB-A devices to begin with?

You can use the USB-C ports with a USB-A adapter.

 

[ACD30"]

You can use the USB-C ports with a USB-A adapter.

Ah, true. Just kind of amazed at how many cables are required.

 

[Burnsey2]

For my fellow Canadian users who don't want to spend $100 on the Club30 cable I got my M1 mini and 30" Cinema display working flawlessly with this adapter from Primecables:

USB-C to Mini DisplayPort/Mini DP Adapter Cable Support 4K@60Hz Resolution (primecables.ca)

Tried a few no name Amazon cables and none worked.

The only issue is sleep to wake does not work, so I don't put my mini to sleep. After a short while it automatically turns off the display which comes back on if you move the mouse, and since the M1 Mini uses so little power and makes no noise sleep mode seems redundant anyway.

 

[EugW]

I got my M1 Mac mini set up with the 30" ACD today. OS is Ventura.

I tried three different setups:

1) CableCreation USB-C to DL-DVI adapter. This dongle appears to be identical to the Club 3D one. My 30" ACD is a 2007 model that supports HDCP. (I bought the CableCreation one only because I'm in Canada and at the time, the CableCreation one was way cheaper. It no longer is for sale though AFAIK.) This works fine EXCEPT:

- Disney+ simply doesn't work. It loads up fine, and if I scrub through the video I can see the thumbnails, but the actual video refuses to play. Either I get a black screen or I get a constant spinning loading animation.
- Prime Video also does not work.
- When I start Netflix sometimes it starts with a black screen. If I just skip forward or back again, then I get the video as desired.

I had assumed this was HDCP-on because Netflix seems to work fine with my Intel Macs (and mostly fine with my M1 Mac). I have since ordered a Club 3D CAC-1510 HDCP-on for further testing. Maybe Netflix working is just a fluke, and HDCP is only strictly enforced with Disney+ and Prime. I haven't tested Apple TV+, because I no longer have a subscription.

I use MonitorControl.app to adjust brightness. The brightness scale goes from 0 to the middle of the scale. You can increase the brightness level bar past the mid-point, but it doesn't actually make the screen any brighter.

So, 0-50 is an increasing scale, but everything from 50-100 is the same. To increase the brightness further, I have to use the buttons on the side of the monitor.

Sleep and wake work fine.


2) No-name USB-C to mini-DP adapter, with Apple mini-DP to DL-DVI adapter. This works fine with my 2017 12-inch MacBook and with my 2017 iMac, but does NOT with the M1 Mac mini. I just get nothing at all.


3) Cable Matters USB-C to Multiport Adapter --> DP to mini-DP dongle --> Apple mini-DP to DL-DVI adapter. This works fine:

- Disney+ works perfectly. Yay!
- Prime Video also works. Yay!
- Netflix occasionally still does start with a blank screen until I skip forward or back but I could live with that.
- However, this appears to have sleep problems.

Note: Although the Cable Matters dongle has USB 2 ports, I do NOT plug the Apple adapter's USB connector into the Cable Matters dongle. If I do that the dongle gets warmer. That concerns me, so instead I plug the USB connector from the Apple adapter into an iPhone charger.

MonitorControl.app changes the brightness scale from 0 to 100, but 100 is about half brightness. It's similar to my result in setup #1) above, but spread over a different scale. ie. 100 in this scale is about 50 in #1) above, both at about the same brightness. In order to go any brighter, you have to use the side buttons. Is there anyway of making this go brighter instead of having to use the brightness buttons on the side? I don't need it way brighter, but sometimes a little brighter would be nice. I tried Lunar with similar results.

I wonder if a different DisplayPort or mini-DP dongle would work. Any recommendations?

 

[ThunderSkunk]

Oh jeez. I just found out some big news, so I’m now bundling up my ACD’s to sell in the classifieds with all their Belkin FireWire hubs & Apple Dual link/FW/usb-c adapters, but this thread is making me second guess whether they’ll actually work with the new M-series macs. I guess I’ll have to pick up an m1 Air after all, just to sell these... uyyyy.

…but yeah, I’m def keeping one for old times sake, bc these things are pure pleasure to have in a room.

 

[EugW]

I tried three different setups:

1) CableCreation USB-C to DL-DVI adapter.

---snip---

Sleep and wake work fine.

Actually, I had noticed that while sleep works fine and the Mac wakes up fine, there is a delay of several seconds before I finally see the login screen, which is actually slower than my Intel Macs.

However, it turns out the M1 isn't slow to wake. It's the monitor. There is a several second lag between computer wake up and monitor wake up with this particular M1 + CableCreation dongle setup.

I confirmed this by setting the M1 Mac mini to use Apple Watch login. Now, when I wake the computer, it auto-logs in from sensing my Watch, and by the time the screen actually turns on, it's already at the desktop. I don't even see the login screen.

I've ordered the Club 3D CAD-1510 HDCP-on USB-C to DL-DVI adapter, so we'll see if it behaves the same way. (The main reason I ordered it is to get Disney+ and Amazon Prime Video streaming though.)

2) No-name USB-C to mini-DP adapter, with Apple mini-DP to DL-DVI adapter. This works fine with my 2017 12-inch MacBook and with my 2017 iMac, but does NOT with the M1 Mac mini. I just get nothing at all.

Does anyone have a specific recommendation for a USB-C to mini-DP adapter for this setup? If not mini-DP, how about USB-C to DP?

Below are some of the options I was looking at on Amazon (Canada). I've ordered the Cable Matters 8K one in the meantime. It lists Multistream support, but people mention in the comments that it gets quite warm with some usage. Somebody in the comments mentioned it uses a Realtek Billboard chipset, if that helps.

 

[EugW]

OK. I have completely given up on trying to get DisplayPort from the M1 Mac mini working with the 30" ACD. Many dongles simply don't work, but those that do all have wake from sleep issues. Below are various tested dongles connected through Apple's mini-DP to DL-DVI adapter to the 30" ACD:

1. No name USB-C to DP dongle. Simply doesn’t work. Black screen.

2. No name USB-C to mini-DP dongle. Sometimes works, but unreliable. Wake from sleep doesn't work.

3. Cable Matters multifunction hub with 4K DisplayPort. Wake from sleep often didn’t work.

4. Cable Matters 8K DisplayPort dongle. Seemed to work initially but then wake from sleep stopped working within a day. Ran very warm.

5. Plugable 4K DisplayPort dongle. Screen shows up fine, but wake from sleep never worked.

6. Club 3D 8K DisplayPort dongle. Black screen, and the M1 Mac mini doesn't even sense the screen. The same is true with my 12" MacBook. I know the dongle isn't defective, because it works perfectly fine from the 12" MacBook hooked up to my 2010 iMac through Target Display Mode.

The only way to get wake up the computer is to unplug the dongle and plug it back in. This IMO is a dealkiller, and really unfortunate because the dongles that work seem to pass HDCP just fine so I can stream Disney+ and Prime Video perfectly.

The other little oddity is that if you use MonitorControl.app to control brightness from the keyboard, you can only go to about half brightness. To go brighter you need to use the buttons on the side of the monitor.

---

So, that leaves me with my two active USB-C to DL-DVI dongles:

CableCreation USB-C to DL-DVI dongle. Works. Apparently this is HDCP off, because Disney+ and Prime Video don't work with this in Safari. Interestingly though, Netflix works up to 1080p. Actually, if you use Chrome, you can get (720p?) HD video with Disney+, and SD video with Prime Video.

Club 3D USB-C to DL-DVI with HDCP-on. Works, including with Disney+ and Prime Video, but only if I use my 12" MacBook. If I use my M1 Mac mini, Disney+ and Prime Video do not work. Again 1080p Netflix works fine. So apparently, the combination of Intel + 30" ACD works fine for HDCP, but M1 Mac mini + 30" ACD means HDCP is borked.

For both of these dongles, there is a 4-8 second delay before the screen wakes up from sleep, but that's acceptable.

Also, both of these dongles work on my Plugable Thunderbolt/USB4 Hub, so I don't have to waste a port on my M1 Mac mini.

 

[EugW]

First I purchased a generic AUD$35 USB-C -> Mini DisplayPort Adapter (Comsol brand https://www.comsol.com.au/KCMDAD, but appears to be a rebadged product) and a generic USB-C -> USB-A adapter to power the Apple adapter. This worked, however the only downside was it would not wake from sleep without replugging the display cable.

I have just acquired an A-Logic Ultra USB-C Dock Plus v2 hub with a Mini DisplayPort, for AUD$159.
https://www.jbhifi.com.au/products/alogic-ultra-usb-c-dock-plus-v2
It works, and supports 100W USB-PD. With a USB-PD 3.0 compliant cable it charges the laptop nearly as fast as the MagSafe cable (both via the Apple 140W charger). It reports various VIA chipsets (and Realtek network chipset)

However, when I plug it in, the 30" Cinema Display flickers on and off every second or so, up to 10 times. It's quite disconcerting, but once it stops the monitor stays connected and the image is consistent. (This may be an issue with the 12 yr-old Cinema Display power brick.)

The A-Logic also seems to wake the Cinema Display from sleep, unlike the cheap USB-C -> mDP adapter. But I will need longer term data to confirm this.

But, the hub/dock gets super HOT. And stays HOT, irrespective of whether the laptop is charging, or even whether the hub/dock is plugged into the charger. It gets hot even when nothing is connected to it, just plugged into the Macbook. I may have to return it, as the temperatures seem abnormally high. There's a thread here discussing similar issues https://forums.macrumors.com/threads/usb-c-hub-gets-very-hot-on-macbook-pro-2020.2238109/

How has this adapter kept up in the last couple of years? As you can see from my previous post above, I have major problems with wake from sleep with DisplayPort.

Also, how do you adjust brightness? Side buttons? Using a DisplayPort dongle, I can only go up to about half brightness if I use MonitorControl.app.

 

[joevt]

MonitorControl.app changes the brightness scale from 0 to 100, but 100 is about half brightness. It's similar to my result in setup #1) above, but spread over a different scale. ie. 100 in this scale is about 50 in #1) above, both at about the same brightness. In order to go any brighter, you have to use the side buttons. Is there anyway of making this go brighter instead of having to use the brightness buttons on the side? I don't need it way brighter, but sometimes a little brighter would be nice. I tried Lunar with similar results.

I used my AllRez command to look at the DDC info for my Apple Cinema HD 30" Display from my Mac Pro 2008.

Max brightness using external buttons

IOFBCopyI2CInterfaceForBus = { id:0x100000000 busType:I2C transactionTypes:(No,Simple,DDCci,Combined,DisplayPort,) commFlags:(0?,UseSubAddress,) };
EDID from E-DDC (old method) = 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
EDID from E-DDC              = 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
Timing Report = { 6e064e022648178984 : { status:(+hsync,-vsync) 98.00kHz 60.25Hz } };
VCP Capabilities = {
    (last byte at offset 126 is not null)
    VCP string = "prot(monitor) type(LCD) model(Cinema HD Display) cmds(01 02 03 06 07 0C E3 F3) VCP(02 05 10 52 B6 C0 C9 CA D6 DF) mccs_ver(2.0)";
    VCP parsed = {
        prot = "monitor";
        type = "LCD";
        model = "Cinema HD Display";
        cmds = {
            01: VCP request
            02: VCP reply
            03: VCP set
            06: Timing reply
            07: Timing request
            0C: Save current settings
            E3: Capabilities reply
            F3: Capabilities request
        };
        VCP = {
            02: New Control Value = { RW, NC, 0000020002:{ ?New control value(s) have been changed (see FIFO 52h) }, },
            05: Restore Factory Luminance / Contrast Defaults = { WO, NC, },
            10: Luminance = { RW, C, 0000FF00FF:{ 100.0% }, },
            52: Active Control = { RO, NC, 0000FF0010:{ 10: Luminance }, },
            B6: Display Technology Type = { RO, NC, 0000080003:{ Transducer: LCD (active matrix), Technology Implementation: Reserved, must be ignored }, },
            C0: Display Usage Time = { RO, C, 00FFFFFB27:{ ?64295 hours }, },
            C9: Display Firmware Level = { RO, C, 01FFFF0114:{ 1.20 }, },
            CA: OSD / Button Control = { RW, NC, 00FFFF00FF:{ ?The display cannot supply this information. The display does not support Host control of its Power function and may NOT report Power “events”. }, },
            D6: Power Mode = { RW, NC, 0000040001:{ DPM On, DPMS On }, },
            DF: VCP Version = { RO, NC, 00FFFF0200:{ 2.0 }, },
        };
        mccs_ver = "2.0";
    } // VCP parsed
}; // VCP Capabilities
DisplayPort = {
    (00000h: dpErrReadDpcd result:paramErr)
}; // DisplayPort

Max brightness using Monitor Control.app

            10: Luminance = { RW, C, 0000FF0064:{ 39.2% }, },

The VCP indicates the max value (255) and the current value (100) for Luminance (called Brightness by MonitorControl.app)
I don't know why MonitorControl.app thinks the max is 100 but the source code is available to look at.

You can override the max Luminance:
MonitorControl.app -> Displays -> Cinema HD -> Show advanced settings -> Brightness -> DDC max -> 255.